This review provides a detailed comparative summary of the recent and current research activities in the area of two-phase anaerobic digestion processes. The acid phase and the methane phase are first evaluated, individually, from microbiological, kinetic and modelling, process optimization, operation and control, inhibition, and toxicity points of view. The overall process performance is subsequently evaluated as a whole. Finally, areas requiring further research are determined. (C) 2002 Society of Chemical Industry.